1. Field of the Invention
Embodiments of the invention generally relate to a method and apparatus for silicon oxide deposition on large area substrates.
2. Background of the Related Art
Thin film transistors (TFTs) are conventionally made on large area glass substrates or plates for use in monitors, flat panel displays, solar cells, personal digital assistants (PDAs), cell phones and the like. Many TFT manufacturers utilize large area substrates for TFT fabrication with dimensions exceeding 550 mm by 650 mm, with a demand for even larger sizes. It is envisioned that these dimensions may exceed 4.0 square meters in the near future.
TFTs are made in a cluster tool by sequential deposition of various films including amorphous silicon, doped and undoped silicon oxides, silicon nitride and the like in vacuum chambers typically disposed around a central transfer chamber. TFTs generally comprise two glass plates having a layer of liquid crystal material sandwiched therebetween. At least one of the glass plates includes at least one conductive film disposed thereon that is coupled to a power supply. Power supplied to the conductive film from the power supply changes the orientation of the liquid crystal material, creating a pattern such as text or graphics seen on the display. One fabrication process frequently used to produce flat panels is plasma enhanced chemical vapor deposition (PECVD).
Plasma enhanced chemical vapor deposition is generally employed to deposit thin films on a substrate such as a flat panel or semiconductor wafer. Plasma enhanced chemical vapor deposition is generally accomplished by introducing a precursor gas into a vacuum chamber that contains a substrate. The precursor gas is typically directed through a distribution plate situated near the top of the chamber. The precursor gas in the chamber is energized (e.g., excited) to form a plasma by applying RF power to the chamber from one or more RF sources coupled to the chamber. The excited gas reacts to form a layer of material on a surface of the substrate that is positioned on a temperature controlled substrate support. In applications where the substrate receives a layer of low temperature polysilicon, the substrate support may be heated in excess of 400 degrees Celsius. Volatile by-products produced during the reaction are pumped from the chamber through an exhaust system.
One of the obstacles in depositing films, particularly silicon oxides formed from TEOS precursors, is the long time required to deposit a predetermined thickness of film on the surface of larger size substrates. In particular, deposition rates slow exponentially as process gases cannot be provided to the chamber at a rate that allows commercially practical deposition rates. For example, conventional vaporizers utilized to convert liquid TEOS into TEOS vapor suitable for CVD processes are limited to about 10 g/m and correspondingly limit deposition rates to about 1500 to about a maximum of 2500 Å/m in typical processes. The lack of generators suitable for providing high volumetric flows of process gases (i.e., flows in excess of 15 g/m) is a major obstacle for commercially practical silicon oxide deposition on next generation size large area substrates.
Further, TEOS vaporizers, such as conventional TEOS bubblers utilized in many large area substrate CVD applications, also tend to generate and entrain liquid droplets at their upper end of operation, which is generally limited to about 10 g/m. Droplets entering the processing chamber may contaminate the substrate and/or result in process variation. As the size of large area substrates commands a substantial investment in material and processing costs, excessive defects due to droplets or inadequate precursor gas generation are unacceptable. Moreover, droplets entrained in the gases entering the processing chamber result in prolonged vacuum pump-down time. For example, conventional large area substrate CVD systems encounter pump-down times of about 23–30 seconds for conventional vaporizers producing 5 g/min TEOS and about 30–34 seconds for conventional vaporizers producing 10 g/min TEOS. Minimization of the pump-down time is highly desirable as it would directly result in increased substrate throughput.
Therefore, is a need for a method and apparatus for generating TEOS vapor (among other precursors or process gases) for depositing dielectric material at a rate of at least 2,000 Å/m on large area substrates.